1. Field of the Invention
The present invention relates to iontophoretic devices for delivering a drug or medicant to a patient transdermally, i.e., through the patient's skin, and more specifically relates to an iontophoretic device having one or more segmented electrodes and current delivery circuits therefor.
2. Description of the Prior Art
Conventional iontophoretic devices, such as described in U.S. Pat. No. 4,820,263 which issued to Richard Spevak et al., for delivering a drug or medicant transdermally, basically consists of two electrodes--an anode and a cathode. Electric current is driven from an external supply into the patient's skin at the anode, and back out at the cathode. Most modern iontophoretic devices are powered by a constant current source to ensure that the current is kept at a desired level despite differences in skin impedance among individuals.
Electrode structures for iontophoretic devices generally have an active electrode (for example, the anode) which delivers the ionic medication, and an indifferent electrode (for example, the cathode) with an electrolyte. The active and indifferent electrodes are connected to the skin forming a circuit. The current passes from a power source through one of the electrodes to the skin, through the skin and into the subdermal tissue, back out through the skin at a separate location and then through the other electrode. Regulation of the rate of drug delivery has been controlled by the amount of current flow.
It has been found in such iontophoretic devices that skin irritation is related to the current density of the applied current. Densities below 200 .mu.a/cm.sup.2 are considered as generally being non-irritating. Current densities above that figure are often associated with skin irritation.
A typical iontophoresis system may include electrodes (an anode and a cathode) which are each 5 square centimeters in size through which a total current of 1 milliamperes is driven. At such values, an average current density of 200 .mu.a/cm.sup.2 is applied to the skin.
If the skin varies considerably in resistance over the area of the electrodes, the peak current densities in different areas of the skin under the electrodes may be considerably higher than the average current density. It has been found that these variations in current density may be as much as 5 or 10 to 1, that is, a more conductive skin area can run at a current density of as much as 10 times the average current density, causing skin irritation or burns in that more conductive area of the skin.
Various electrode arrangements have been suggested to avoid burns and irritation where the iontophoresis or transdermal drug delivery takes place. For example, U.S. Pat. No. 4,211,222 which issued to Robert Tapper, discloses an iontophoretic electrode array for use in transdermal transport of ionic medicants which includes a plurality of positive and negative electrodes for establishing electric field lines in an area and for transmission of ions along the lines. Also, U.S. Pat. No. 4,416,274 which issued to Stephen C. Jacobsen et al. discloses an iontophoretic bioelectrode which includes a receptacle for holding an ionic medication formed with a plurality of separate cube-shaped compartments to help control the distribution of the medication or drug over the area of the bioelectrode. However, such conventional electrode arrays do not prevent excessive current from being drawn through the patient's skin from portions of the electrode contacting areas of the skin which have a significantly lower skin impedance than at other areas.